The present invention relates to a slack adjuster which is principally, but not exclusively, for use in rail transport vehicles. It will be convenient to describe the invention in relation to its particular application to rail bogies, although it is to be appreciated that the invention could have wider application.
Slack adjusters are employed in rail bogie braking systems that utilise sacrificial brake or friction linings, to compensate for wear of the friction lining over time. Slack adjusters operate by progressively shifting the brake shoes on which the friction lining is mounted, toward the braking surface that the friction lining engages during braking. Depending on the particular style of slack adjuster and the arrangement to which it is applied, the adjuster may either act directly or indirectly on the brake shoes.
It is known to apply linear slack adjusters to rail bogies. These devices employ spring pressure to take-up and remove slack in brake levers and linkages upon lining wear. However, linear devices employ large springs that store energy and therefore they can be very dangerous during assembly and normal maintenance. Additionally, linear slack adjusters extend through the central bolster of a rail bogie and require a large opening for that purpose.
It is an object of the present invention to provide a slack adjuster which overcomes or at least alleviates the above disadvantages. It is a further object of the invention to provide a rotary slack adjuster of robust construction and of relatively low maintenance.
According to the present invention there is provided a rotary slack adjuster for use in a brake system of a vehicle for compensating for wear in brake friction lining, said adjuster including an input arm for connection to an actuating piston of a brake actuator, an output arm for connection to an output rod, and a rotatable member, coupling means being provided for coupling said input arm, said output arm and said rotatable member together for mutual rotation during brake actuation in each of a first direction and a second opposite direction, said rotatable member being confined to rotate between end limits in each of said first and second directions, wherein said coupling means permits said input arm and said output arm to further mutually rotate in said first direction relative to rotatable member upon said rotatable member rotating to said end limit in said first direction, and said coupling means being arranged so that upon relative rotation between said input and output arms and said rotatable member exceeding a predetermined amount, said input and output arms are restrained against return rotation relative to said rotatable member and are thus repositioned relative to said rotatable member, and wherein said coupling means permits said input arm to further rotate in said second direction relative to said output arm and said rotatable member upon said rotatable member rotating to said end limit in said second direction at which end limit said coupling means prevents further rotation in said second direction of said output arm, and said coupling means being arranged so that upon relative rotation between said input arm and each of said output arm and said rotatable member exceeding a predetermined amount, said input arm is restrained against return rotational movement relative to said output arm and said rotatable member, and said input arm is thus repositioned relative to said output arm and said rotatable member, wherein in use, a brake system including said slack adjuster is arranged so that the respective repositioning of said input and output arms results in a shift in the brake friction lining toward the braking surface, so compensating for wear in said friction lining.
A slack adjuster according to the present invention is of rotary operation and has substantial mechanical capacity which makes its application ideal for rail bogies.
The present invention further provides a brake system for a vehicle, including a slack adjuster of the aforementioned kind wherein said input arm is connected to the actuating piston of a brake actuator and said output arm is connected to an output rod, said slack adjuster being supported on a first movable structural member which includes a pair of brake heads, said brake actuator being supported on a second movable structural member spaced from said first structural member and which includes a pair of brake heads, separation of said first and second movable structural members through said slack adjuster resulting in said pairs of brake heads engaging braking surfaces for braking a moving vehicle.
In a preferred arrangement, the input and output arms comprise an annular body defining a central opening and a radial extension depending from the annular body. In each of the arms, the radial extension facilitates connection between said arms and the actuating piston or the output rod. It is preferred that the rotatable member also be formed as an annular body defining a central opening, although the rotatable member does not require a radial extension.
It is preferred that the input and output arms and the rotatable member are adjacently located for coaxial rotation. In this arrangement, the slack adjuster may include a central post about which each of the input and output arms and the rotatable member are disposed in an adjacent and coaxial manner. The post may be stepped lengthwise in each of three spaced apart locations in order to support each of the input and output arms and the rotatable member.
In a preferred arrangement, the input arm and the output arm are connected through ratchet teeth in a manner that permits the required mutual and relative rotation in the respective first and second directions of rotation. A toothed ratchet connection can also be employed between the output arm and the rotatable member. In this arrangement actual compensation for slack occurs only when the amount of slack permits the relative rotation between the input and output arms to ratchet at least a single tooth. Any number of teeth can be employed in the ratchet connection, but the fewer teeth employed, means that greater slack will be required for a single ratchet movement. It is considered that a ratchet connection of at least 60 teeth is appropriate.
In a preferred arrangement, the input arm is connected to the push rod of a brake actuator, which is mounted on a movable structural member that includes a pair of brake heads. In this arrangement the slack adjuster can be mounted on another movable structural member, spaced from the first structural member, and which also includes a pair of brake heads. Each of the brake heads includes a friction material for engaging the braking surface of the wheels of a bogie and engagement of the friction material with the braking surface of the wheels occurs by separating the structural members on which the brake actuator and the slack adjuster are mounted. An arrangement of push rods moving linearly under brake actuation, permits this separation. In the above arrangement, the output arm of the slack adjuster, is connected to a second push rod, which extends into connection with the first structural member. Actuation of the brake actuator results in extension of the first push rod, which, through its engagement with the input arm, causes the input arm to rotate, and through the connection between the input arm and the output arm, the output arm is also rotated. The connection between the output arm and the second push rod causes the second push rod to move linearly in the opposite direction to the extension movement of the first push rod and the movement of the two push rods causes separating movement of the respective structural members. That movement permits engagement between the friction material and the wheels for braking the bogie.
Upon release of the brake actuating force, the push rods retract and the friction material is withdrawn from the bogie wheels. The correct spacing at rest between the friction material and the braking surface is termed the xe2x80x9cblock clearancexe2x80x9d. When the friction material wears, the block clearance increases and it is the operation of the slack adjuster to reposition the brake heads to return the block clearance to the preworn spacing. In the present invention, upon brake actuation, the input and output arms mutually rotate, and in the presence of slack, extra rotation is required to move the push rods and thus the structural members, the extra distance to take up the slack. That extra rotation is permitted, but only with relative rotation between the output arm and the rotatable member, as the rotatable member is confined to movement between predetermined end limits, provided for example, by suitable abutment means to rotate in either direction only an amount which corresponds to the block clearance.
When the brake actuation is released, the input and output arms and the rotatable member, each rotate in the reverse direction. The abutment means that limits rotation of the rotatable member also applies in the reverse rotation direction and through the connection between the rotatable member and the output arm, upon rotational movement of an amount corresponding to the block clearance, the output arm is also prevented from further reverse rotation. The input arm is not however so prevented and can continue to rotate relative to the output arm. That relative rotation continues for an amount equal to the slack to be taken up and repositions the input and output arms. If sufficient slack is present, then the input and output arms will ratchet and therefore will be permanently repositioned relative to one another. That repositioning affects the position of the push rods and also the structural members which, as a result, are spaced further apart bringing the brake heads closer to the bogie wheels and thus maintaining the correct block clearance.
The slack adjuster of the invention does not require energy storage and therefore does not need to employ the large springs of the prior art. Additionally, although the push rods of the preferred arrangement still extend through the central bolster, the opening required is smaller than in the prior art. The invention is also of a more simple construction compared to the prior art and is considered to be very robust and have low maintenance requirements.